All that was left was a triangular piece of metal.
Why is it possible to have two verbs here? Is one “was” a verbal? Is there two clauses here?
I think "All that was left" is a noun phrase, and used as the subject of the sentence here.
Think of it like this:
[All [that was left]] was [a triangular piece of metal].
"All that was left" is a noun phrase, and is the subject of the main clause. "Was" is the verb of the main clause, and "a triangular piece of metal" is the complement of the verb.
"That was left" is a relative clause (a subordinate clause starting with a relative pronoun). Within the relative clause, the pronoun "that" is the subject, "was" is the verb, and "left" is the complement of the verb.
So there is a subordinate clause embedded within the subject of the main clause.
All that was left was a triangular piece of metal.
All is a subject(S) and was is a finite verb (real verb)
"that was left" a relative pronoun clause which has "All" as an antecedent.
In this relative clause(that was left), "that" is a subject and it is called "nominative or subjective case of relative pronoun"; here the relative pronoun clause "that was left" modifies the "All".